The present invention relates to a laser control technique for an image forming apparatus such as a laser beam printer (LBP) or a copying apparatus, using electrophotography.
Heretofore, in a laser-driver circuit for an image forming apparatus, a method wherein an output of a laser beam is detected in a photodetection period during one scanning and fed back, and a drive current is maintained for one scanning period.
A laser has such a characteristic that it causes self-heating, thus requiring a larger quantity of electric current or obtaining a predetermined quantity of light with a higher temperature. As a result, the predetermined light quantity cannot be obtained only by supplying a predetermined current at all times, thus resulting in a lowering in image qualities. Accordingly, in order to obviate the lowering in image quality, an APC (auto power control) scheme is employed for each one scanning to control a current quantity so as to provide a certain light emission characteristic for each scanning.
Hereinbelow, a specific control method will be described with reference to FIG. 17.
The image forming apparatus of the aforementioned type, as shown in FIG. 17, employs a laser chip 43 consisting of a laser 43A and a photodiode (PD) sensor 43B, and two current sources comprising a bias current source 41 and a pulse current source 42 are adopted to the laser chip 43, thus improving a light emission characteristic of the laser 43A. Further, in order to stabilize emission of light from the laser 43A, the bias current source 41 is caused to give feedback by using an output signal from the PD sensor 43B to automatically control a bias current quantity.
More specifically, based on a full lighting signal from a sequence controller 47, when a logic element 70 outputs an ON-signal to a switch 49, a sum of electric current supplied from the bias current source 41 and the pulse current source 42 flows through the laser chip 43. An output signal at that time is inputted into a current-voltage converter 44, amplified by an amplifier 45, and inputted into an APC circuit 46. The APC circuit 46 supplies a control signal to the bias current source 41 so that the inputted voltage reaches a target voltage. This circuit scheme is called an APC circuit scheme which is generally used as a circuit scheme for driving a laser at present.
The resultant laser beam controlled to have a predetermined light quantity is used for image formation by turning the switch 49 on and off based on data modulated in a Pixel modulation unit 48 (e.g., Japanese Laid-Open Patent Application No. Hei 05-130332).
However, even if a light quantity of laser beam by using the conventional APC circuit scheme is kept constant, there is still room for improvement depending on qualities of photosensitive members used. This is attributable to a nonuniformity in thickness of a film formed on the surface of the photosensitive member. More specifically, even in the case where the same amount of light quantity of a laser beam is irradiated over the entire surface of the photosensitive member shown in FIG. 16(a), a resultant surface potential does not become constant, thus causing a potential irregularity in some cases. For instance, in the case where a surface potential distribution in a main scanning direction results in the one shown in FIG. 16(b) or a surface potential distribution in a subscanning direction results in the one shown in FIG. 16(c), an unevenness in density occurs in the resultant image, thus being desired to be further improved. However, on the other hand, it is very difficult to provide the photosensitive member surface with a uniform film thickness.
A first object of the present invention is to provide a laser beam control method for an image forming apparatus and an image forming apparatus, capable of improving image qualities by remedying a potential irregularity at the surface of a photosensitive member in the image forming apparatus.
A second object of the present invention is to provide a laser beam control method for an image forming apparatus and an image forming apparatus, capable of correcting two-dimensionally a potential irregularity with a small amount of memory utilization.
In order to achieve the first object, according to a first aspect of the present invention, there is provided a method of controlling a laser beam for an image forming apparatus of the type wherein an electrostatic latent image is formed by irradiating a charged surface of a photosensitive member with a laser beam depending on a picture signal and is visualized with a recording agent, followed by transfer onto a recording medium to form an image, the method comprising:
a step of controlling predetermined drive currents supplied from a plurality of current sources including a first current source and a second current source so that a drive current supplied from the first current source is controlled to be constant during one scanning and a drive current supplied from the second current source is switched during one scanning at a plurality of points on the surface of the photosensitive member, based on light quantities of a laser beam at the time of supplying predetermined drive currents from the first and second current sources to predetermined areas, respectively, in a scanning direction of the laser beam at the surface of the photosensitive member.
By using the laser beam control method, a drive current supplied from the first scanning direction is made constant during one scanning and a drive current supplied from the second scanning direction is switched from point to point with respect to a plurality of points on the photosensitive member surface, so that a potential irregularity at the surface of the photosensitive member can be improved to realize image formation with improved image qualities.
In order to achieve the first object, according to a second aspect of the present invention, there is provided an image forming apparatus, comprising:
an image forming unit for irradiating a charged surface of a photosensitive member with a laser beam depending on a picture signal to form an electrostatic latent image, visualizing the latent image with a recording agent, and transferring the visualized image onto a recording medium to form an image,
a memory for storing a correction value in a main scanning direction of the photosensitive member,
a circumferential position detection unit for detecting a circumferential position irradiated with the laser beam at the surface of the photosensitive member, and
a control unit for setting a target light quantity depending on the detected circumferential position and controlling a light quantity of the laser beam depending on the correction value stored in the memory and the target light quantity.
By using the image forming apparatus, a light amount of laser beam is controlled based on a correction value in a main scanning direction of the photosensitive member and a target light quantity depending on a circumferential position of the photosensitive member, thus further remedying a potential irregularity at the photosensitive member surface to improve image qualities. Further, it becomes possible to accomplish the first object with a small amount of memory usage since a correction value for one line in the main scanning direction is sufficient to accomplish the first object.
In order to achieve the first object, according to a third aspect of the present invention, there is provided an image forming apparatus, comprising:
an image forming unit for irradiating a charged surface of a photosensitive member with a laser beam depending on a picture signal to form an electrostatic latent image, visualizing the latent image with a recording agent, and transferring the visualized image onto a recording medium to form an image,
a detection unit for detecting a position in a main scanning direction and a position in a sub scanning direction, at the surface of the photosensitive member irradiated with the laser beam,
a memory for having combinations of the positions in the main and sub scanning directions at the surface of the photosensitive member as addresses and storing data indicating a target light quantity for each address, and
a control unit for reading out the target light quantity from the memory depending on a position detected by the detection unit and controlling the laser beam so as to provide the target light quantity.
The image forming apparatus includes a memory having an address comprising a combination of positions in main and sub scanning directions at the photosensitive member surface and storing data indicating target light quantities for each address, whereby image qualities are further improved through remedy for potential irregularity of the photosensitive member.
In order to achieve the second object mentioned above, according to the present invention, there is provided an image forming apparatus, comprising:
an image forming unit for irradiating a charged surface of a photosensitive member with a laser beam depending on a picture signal to form an electrostatic latent image, visualizing the latent image with a recording agent, and transferring the visualized image onto a recording medium to form an image,
a first memory for memorizing a correction value in a main scanning direction of the photosensitive member,
a second memory for memorizing a correction value in a sub scanning direction of the photosensitive member,
an arithmetic unit for providing a correction value at a corresponding surface position of the photosensitive member by performing an arithmetic operation of the correction values in the main and sub scanning directions,
a circumferential position detection unit for detecting a circumferential position of the photosensitive member irradiated with the laser beam, and
a control unit for controlling the laser beam depending on the correction value provided by the arithmetical unit and the circumferential position detected by the circumferential position detection unit.
The image forming apparatus can control a laser light quantity so that a potential irregularity is two-dimensionally corrected based on a tendency of potential irregularity in the main and sub scanning directions at the surface of the photosensitive member, thus improving qualities of image. Further, an amount of memory for one line is only required in each of the main and sub scanning directions, so that an amount of memory utilization can be reduced.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.